1. FIELD OF THE INVENTION
The present invention relates to microwave delay lines and more particularly to microwave delay lines having a frequency divider circuit.
2. DESCRIPTION OF THE PRIOR ART
In the past, acoustic delay devices have been used in microwave delay lines for providing a predetermined delay for microwave signals. Acoustic delay devices are useful in microwave delay lines since they are capable of providing relatively long signal delays per unit length of acoustic material. However, prior art acoustic delay devices are not operable beyond an upper frequency limit determined by critical acoustic device dimensions.
A prior art solution to the problem of providing a delay for a signal at a frequency exceeding the upper operating frequency of an acoustic delay device is to reduce the frequency of the input signal by use of a frequency down-converter. As known in the art, a frequency down-converter is a circuit having a semiconductor device, with a non-linear current-voltage characteristic, arranged to mix a first signal or input signal at frequency f.sub.1, with a second signal or local oscillator (L.O.) signal at frequency f.sub.2. The resulting down-converter output signal or intermediate frequency (I.F.) signal is at frequency f.sub.3, or that frequency which is the difference between f.sub.1 and f.sub.2. However, in addition to a difference signal at frequency f.sub.3, down-converters also generate undesired spurious signals resulting from cross modulation which interfere with information carrying input signals.
In addition, prior art frequency dividers of the type described above are not suitable for producing an output signal free from undesired noise or amplitude modulation which may be present in a frequency modulated input signal coupled to the frequency divider. Also, it will be appreciated that for certain delay line applications where space and operating efficiency are critical, the additional power requirements for a local oscillator signal source and the complicated, space consuming, circuitry associated with a frequency down-converter are undesirable.
In the past, frequency division circuits have been limited in capability by semiconductor device constraints. In particular, most logic circuitry has been constructed using silicon semiconductor devices. This has limited the speed of the devices as a result of the electron mobility in silicon and the required electric field for obtaining velocity saturation in silicon.
It has been learned that some semiconductors such as gallium arsenide, indium phosphide and other III-V compounds have a much higher electron mobility than silicon, while requiring a much lower electric field for obtaining electron velocity saturation compared to silicon. Consequently, such devices have a lower delay-dissipation product than do silicon devices. Furthermore, gallium arsenide can be obtained in a semi-insulating form which has excellent dielectric properties even at high microwave frequencies. This means that semiconducting gallium arsenide can be grown homo-epitaxially upon semi-insulating gallium arsenide with no lattice mismatch problems.
In addition to the general advantages of certain III-V compounts such as gallium arsenide over silicon as a semiconductor, there is a physical phenomenon which exists in these compounds but not in silicon which may be used for high speed logic applications. This phenomenon is such that when there is applied in a body of the material an electric field higher than a threshold value determined by the material, a high field domain is formed in the material and travels through the body under the influence of the applied voltage to result in a temporary decrease in current flow through the body. The effect is commonly referred to as the transferred electron effect. Devices that take advantage of the transferred electron effect are called "Gunn-effect" or transferred electron devices (TED's). TED's are two-valley bulk devices and not junction devices. Therefore, TED's do not suffer from speed limitations due to junction capacitance.
The structure and operation of two-valley devices are described in detail in a series of papers in the January 1966 issue of the IEEE Transactions on Electron Devices, Vol. Ed-13, No. 1. As is set forth in these papers, a negative resistance can be obtained from a bulk semiconductor wafer of substantially homogeneous constituency having two energy band minima within the conduction band which are separated by only a small energy difference. By establishing a suitably high electric field across opposite ohmic contacts of the semiconductor wafer, oscillations can be induced which result from the formation of discrete regions of high electric field intensity and corresponding space-charge accumulation, called domains, that travel from the negative to the positive contact at approximately the carrier drift velocity. A characteristic of the two-valley semiconductor material is that it presents a negative differential resistance to internal currents in regions of high electric field intensity. Hence, the electric field intensity of the domain grows as it travels toward the positive electrode.
Solid state oscillators of the "Gunn-effect" type have attracted widespread attention due to their small size and low cost as compared to other available microwave oscillator arrangements, e.g., klystrons, magnetrons, traveling wave tubes, etc. Essentially, such oscillators comprise a small specimen of particular semiconductive material having a multivalley conduction band system and capable of generating current oscillations in the microwave range when subjected to electric fields in excess of a critical, or threshold, intensity E.sub.T. According to the present theory, a high electric field region, or domain, forms within the semiconductive specimen when subjected to electric fields in excess of a critical intensity E.sub.T due to a redistribution of electric fields within the specimen. Such redistribution of electric fields results from a transfer of charge carriers from a high mobility conduction band to a low mobility conduction band under the influence of applied electric fields in excess of the critical intensity E.sub.T. A domain, when nucleated, is sustained and propagated along the semiconductive specimen by electric fields greater than a sustaining intensity E.sub.S, which is less than the critical intensity E.sub.T. The presence of a domain has the effect of reducing the overall conductance of the semiconductive specimen; the magnitude of current flow through the semiconductive specimen varies according to the presence and absence of a domain. Accordingly, a constant voltage of particular magnitude applied across the semiconductive specimen is effective to nucleate and propagate domains in successive, or cyclic, fashion whereby current through such specimen and, hence, along a series-connected load varies periodically in the form of coherent current oscillations. The theory of the "Gunn-effect" has been described more fully in "Theory of Negative-Conductance Amplification and of Gunn Instabilities in `Two-Valley` Semiconductor" by D. E. McCumber et al., IEEE Transactions of Electron Devices, Vol. ED-13, No. 1, January 1966.
The frequency of current oscillations generated by oscillators of the "Gunn-effect" type operated in the travelng domain, or transit-time, mode depends upon the device length L and propagation velocity v of the domains along the active region, i.e., v/L, where v is about 10.sup.7 cm/sec. There is a further requirement for traveling domain oscillations in n-type gallium-arsenide that the product of the ionized donor density, no, and the device length, L, exceed 10.sup.12 cm.sup..sup.-2.
Heretofore, TED's have been used in circuits in which they have been supplied with direct current and have provided a microwave frequency output characteristic of the particular TED dimensions as disclosed in U.S. Pat. No. 3,365,583 to J. B. Gunn, and they have been used in amplifier circuits where they are supplied with an input whose frequency is the same as the characteristic frequency of the TED.
TED's have also been used in logic circuits, primarily as comparators. With regard to logic applications of TED's reference may be made to U.S. Pat. No. 3,594,618 issued to H. L. Hartnagel, "Theory of Gunn effect logic", Solid-State Electronics, Vol. 12, pp. 19-30, 1969; to Toshiya Hayashi, "Three-terminal GaAs Switches", IEEE Elec. Dev., Vol. ED-15, No. 2, pp. 105-110, February 1968; and to T. Sugeta, H. Yanai, and K. Sekido, "Schottky Gate Bulk Effect Digital Devices", Proc. IEEE (Letters) Vol. 59, No. 11, pp. 1629-1630, November 1971.